The present invention pertains to a non-fibrous polymer solution of para-aramid in a mixture of a polar amide solvent selected from N-methyl-2-pyrrolidone, N,N′-dimethylformamide, N,N′-dimethylacetamide, tetramethylurea, and mixtures thereof, water, and an alkali or alkaline earth metal chloride, such as calcium chloride (CaCl2) or lithium chloride (LiCl). The invention further relates to (1) a method of preparing said solution, (2) a method of spinning the polymer solution, (3) a para-aramid pulp, (4) para-aramid paper and (5) para-aramid film made of said solution.
Para-oriented aromatic polyamides which are condensation polymers of a para-oriented aromatic diamine monomer and a para-oriented aromatic dicarboxylic acid halide monomer (hereinafter abbreviated to “para-aramids”) have hitherto been known to be useful in various fields such as fiber, pulp and the like, because of their high strength, high elastic modulus and high heat resistance. Poly(para-phenylene terephthalamide) (hereinafter abbreviated to “PPTA”) is one example of a para-aramid.
Hitherto, PPTA has been produced in polar amide solvent/hexamethylphosphoramide (HMPA) or in polar amide solvent/salt systems. Thus, PPTA is produced by carrying out a solution polymerization reaction in a polar amide solvent. The PPTA is precipitated, neutralized, washed with water and dried, and once isolated as a polymer. Then, the polymer is dissolved in a solvent and can be made into a PPTA fiber by the process of wet spinning. In this step, concentrated sulfuric acid is used as the solvent for spinning dope, because PPTA is not readily soluble in organic solvents. This spinning dope usually shows an optical anisotropy.
Industrially, PPTA fiber is produced from a spinning dope using concentrated sulfuric acid as a solvent, considering the performances as a long fiber, particularly strength and stiffness.
According to the prior process, a pulp is produced by mechanically cutting a PPTA fiber, dispersing the cut fiber in water and fibrillating the dispersed fiber by a mechanical shearing means such as beating or the like, followed by filtration and drying. In such prior process, the steps of polymerization, spinning, and pulp making are completely independent of one another. That is, the step of polymerization uses a polar amide solvent, the step of spinning uses concentrated sulfuric acid as solvent, and the step of pulp making uses water as a dispersing medium. This is economically disadvantageous as an industrial process.
Therefore, it has been attempted to spin the polymer directly into pulp. In U.S. Pat. Nos. 4,959,453 and 5,021,123, a fiber-containing non-pourable gel was prepared. After gelation, the product must be isolated by further dispersing the composition by dilution in a vigorously stirred precipitating medium comprising a non-solvent for the polymer. Spinning of this fibrous gel is very difficult and fiber properties cannot be controlled. Further, it has been disclosed that extrusion must be done under pressure and at high temperature (i.e., 90° C.). Furthermore, it is required to use N-methyl-pyrrolidine in order to obtain pulp-like fibers, as was disclosed in Example A of U.S. Pat. No. 5,021,123.
In U.S. Pat. No. 3,673,143, particularly in Examples 8 and 9, para-aramid solutions were prepared. In Example 9, a chloro-substituted para-aramid was dissolved in N,N-dimethylacetamide (DMAc) without the addition of an alkali or alkaline earth metal chloride. The latter is redundant, because these chloro-substituted para-aramids dissolve well in DMAc. However, this is not the case when unsubstituted, or para-aramids having more than 50 mole % of their aromatic moieties unsubstituted, are used. It is known that these para-aramids are insoluble in most solvents. In Example 8 of this reference, an unsubstituted para-aramid was dissolved in DMAc by adding large amounts of HMPA. HMPA, however, is highly carcinogenic and its use in industrial production of para-aramid polymers is prohibited.
WO 94/24211 discloses a solvent system wherein the toxic HMPA was replaced by substantial amounts of PVP. Although solutions with PVP are good spinnable, their disadvantage is that the polymer is obtained as a mixture of PPTA and PVP, thus the products (fibers, films, etc.) also are composed of mixed polymers. For many applications such mixed polymers are unwanted.
In EP 572,002, pulp was prepared by producing para-aramid polymer in NMP/CaCl2, spinning the fiber, and cutting and refining it into pulp. Although spinning takes place directly from PPTA in a mixture of NMP and calcium chloride, this process has the disadvantage that fibers still have to be spun before cutting and refining. Furthermore, the molecular weight of the polymer solution and of the pulp obtained by such a process is limited, i.e., the polymer has a low relative viscosity, due to the high dynamic viscosity of this solution.